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Home , Cahnite, Willemite

JOURNAL MINERALOGICALSOCIETY OF AMERICA 149

CAHNITE, A NEW BORO-ARSENATE OF CALCIUM FROM FRANKLIN, C. Pnr,acnB,c.NoL. H. BauBn. The here to be describedwas first observedand sketched by Mr. Lazard Cahn about 14 years ago in the form of a few minute implanted crystals. The specimenscontaining them were placed in the hands of the senior author for study and became part of the Holden Collection at Harvard University. The tiny white glassy crystals were very characteristically twinned and their form and angles suggestedstrongly a relation to the barium zeolite edingtonite but material for analysis or for any but the simplest chemical tests was lacking. However, the writer believed that he could show that barium was lacking and that in its stead there was calcium. It was therefore regarded as a calcium edingtonite and the name was proposed for it as a recognition of Mr.Lazard Cahn's indefatigableefforts to preserveand to make known to sciencethe rarer Franklin . This name appeared in the American Minerologist for l92l in the title of a paperl which was neither read nor printed. Thus the matter stood until the spring of. 1926 when Mr. GeorgeStanton of Franklin rediscovered this mineral in moderate abundance in veins in massive ore, in the northern part of the mine. Its identity remained at first concealedas the newly found material was poorly crystallized. The junior author had es- tablished its peculiar chemical nature before the characteristic twinned crystals were again found. Spectroscopic examination of one of the original crystals was then made at the Palmerton laboratory of the New Jersey Company by Mr. Nitchie which established the complete chemical identity of the two finds. Cahnite crystallizes in the tetragonal system with sphenoidal symmetry. Measurements were made both on the first-found crystals and on the later ones, the quality of both lots being poor. While the measurements of difierent crystals agree fairly closely in value, all are unsatisfactory by reason of the etching of practi- cally all crystal faces. The crystals are generally small, rarely exceeding2 or 3 mm. in diameter and are always characteristically

I Holdenite and Cahnite, two new minerals from , New Jersey' Am,. M ineral. 6, 39, 1921. 150 THE AMEKICAN MINERALOGIST

twinned with the appearance, essentially, of interpenetrating tetrahedrons twinned with parallel axes as in sodalite. This mode of twinning causes the positive and negative sphenoids to be brought to parallel position in the two individuals and they therefore reflect simultaneously. The larger, more brilliant sphenoidhas been taken as the positive form p(111): the smaller facesas the negative form o(111). The better developedprisrn is truncated by the perfect prismatic ,m, (lI0). The angular values given by the two best measured crystals follow. Crystal1. Formsa(100), p(lll), o(tlt) ObservedAngles Calculated Angles Opep I oo"02, 90. oo, I o\ go 02 90 oo I oo.oo, 9oooo, Irsooz eooo I 44Bs '\-,[ 4ro2 \ +soo 4roz 44 31 41 02 l 1-r3s ls 4r 02 \ 13s oo 4r oz o[*135 16 41 02 J

Crystal 2. Formsa(100), Cleav. (110), p(111), 0(111)

OpO o I 00' 0o' 90o 00, \ oo" 00' 90' 00' 1 e000 e000 I .t 13s 00 90 00 13s 00 90 00 creav.l-135 \ 00 90 00 J (110) 44 52 41 03 45 00 41 02 i 135 52 4l 01 135 00 41 0

The average value of the sphenoid angles, 6 45", p 41" 42t yield,s the axial ratio a:c:1:0.615. po:0:615 Other signals were obtained from some of the crystals but they were variable in position and were undoubtedly from rounded etched faces, yielding no simple indices. The figure shows an idealized twin crystal. One group approxi- mated to this development but generally the twins are far less regularly formed. Characteristic for all however are the reentrant angles appearing on the prism face o, which is common to the two parts of the twin; and the rectangular crossing of the two sphenoid edgesof p. The sharpnessof the form is commonly lost by the rounding and facetting due to etching. In the latest find JOURNAL MINERALOGICAL SOCIETY OF AMEMCA 151 of cahnite the etching is so extensive that the twin crystals are reduced to plates parallel to the basal plane, still however, showing reentrants on the bright prism faces.

Cahnite. Idealized Twin Crystal.

Cahnite is white and transparent with a notable glassy . The optical characters, determined by Mr. Ber'man, are as follows: Uniaxial, positive. strong, Refractive indices,a: 1.662; e: 1.663 The double refraction is therefore very low. Due to the low birefringence and considerable dispersion of cahnite it exhibits abnormal interference colors somewhat similar to the same phenomena shown in torbernite as described by Bowen.2 This striking effect makes the mineral easily recog- nizable under the microscope. The hardness of cahnite is about 3 and the specific gravity 3.156. The very perfect cleavageparallel to the first order prism (110) is noteworthy. The appearanceof the mineral is very much like that of the barite with which it is sometimes associated. Material for the analyses was purified by hand picking, and for the largest sample, by the use of heavy solutions.

2 Abnormal Birefringence of Torbernite, Am. four. Sri., Vol.48, p. 195, 1919. T52 THE AMERICAN MINERALOGIST

1. Analysisof a sarnpleof cahnite of .1 gr. with slight impurities of hedyphane and calciteby L. H. Bauer. 2. Analysis of a sample of cahnite of .26 grams with slight impurity by L. H. Bauer. 3. Analysis of a very pure sampleof .5 gram by L. H. Bauer, 4. Molecular ratio of 3. 5. Calculatedcomposition of 4CaO.BzOa.AszOo.4HzO.

r. 2. 3. 4. 5. CaO s8.27 37 ls s7.62 .67r:4x.r68 37.64 BzO, 10.14 11.64 11.86 .169:rx.l67 11.74 AszOs 36.79 37.47 38.05 .166:tx .167 38.54 HrO 1r.75 11.78 12.42 .689:4X.172 12.08 PbO 1.15 trace Mgo 0.24 ZnO trace 1.58 COz trace

99.99 99.60 99.95 100.00

The determination of was made by Chapin's modified distillation method on a separate portion in each case. The close agreement of the three analyses made on different samples of cahnite is striking. The molecular ratio leads to the simple formula 4CaO.BzOr.AszOr.4H2Oor Ca+BrAsrOrz.4H2O. Cahnite is thus an entirely new type of chemical compound among minerals. Cahnite fuses quietly at about 3 yielding the green flame of boron. It is easily and completely soluble in dilute hydrochloric acid. In the closedtube, heatedalone it yields water and becomes opaque but does not fuse; heated with potassium carbonate and carbon it yields an mirror. Cahnite has been found only at Franklin. The first crystals occurred on specimens which probably came from the Parker Shaff They present two distinct types of paragenesis. In one cahnite is implanted on the walls of cavities in beautifully crystal- lized axinite together with barite and pyrochroite. In the second type cahnite, calcite, and olive green crystals are im- planted on massive friedelite and barite or on garnet. One crystal on such a specimen, exceptional in being untwinned, is also the Iargest crystal of cahnite yet seen,measuring 6 mm. in diameter. JOURNAL MINERALOGICALSOCIETY OF AMERICA 153

The specimensfound by Mr. Stanton in 1926 came from pillar No. 229 north, 36 feet above the 700'level of the Franklin Mine. The specimensare always associatedwith axinite which constitutes veins with narrow open cavities. Resting on this are crystals, barite, hedyphane, sometimes in crystals, and willemite of either the usual prismatic habit or in thin plates with the base dominant. The cahnite is later than and implanted upon all these associated minerals. The only mineral to form after cahnite is da.tolite in the form of a coating of fibrous nature like what has been called botryolite at Arendal, Norway. This coats all other minerals in the veins although the cahnite crystals are generally free from it and clearly belong to the same period of deposition. Cahnite has also been found in a neighboring part of the mine associated only with rhodonite, on crystals of which it is im- planted. Still another association of the new mineral has come to light as this article is being written. Specimensfrom the picking table show small drusy cavities in Iined with dodecahedral crystals of garnet. The garnet is pink on the free surfaces, and shows successivelywhite and yellow bands towards the walls of the cavities. Tiny glass-clearcrystals of cahnite are implanted on the garnet and show to the minutest detail the complete symmetry of the twin figured above. The only associatedmineral is a brown to light yellow biotite mica in slender, long prismatic crystals projecting into the cavities. The authors are indebted to Mr. Stanton for both gifts and oans of cahnite specimensfor analysis and study.